Because of their ability to deposit material or etch microscopic features with great precision, FIB systems are used in a variety of applications, including processing integrated circuits, trimming thin film heads for disk drives, processing Micro Electro Mechanical Systems (MEMS), and preparing samples for viewing in Transmission Electron Microscopes (TEMs). These applications and others continually demand increased speed and accuracy for creating ever smaller, more elaborate, microscopic features on solid surfaces. In many applications, many cubic microns of material must be removed or added in seconds or minutes.
The standard methods for creating microscopic features involve scanning a finely focused ion beam in a pattern over the target surface to mill, etch, or deposit material. Milling involves the direct removal of surface material by the impact of ions in a process called sputtering. In FIB deposition, a gas, typically including organometallic compounds, is directed toward the impact point of the FIB on the target surface. The gas decomposes in the presence of the ion beam to add material to the target surface. Ion beam assisted deposition processes are described, for example, in U.S. Pat. No. 4,876,112 to Kaito et al. for “Process for Forming Metallic Patterned Film,” and U.S. Pat. No. 5,827,786 to Puretz for “Charged Particle Deposition of Electrically Insulating Films.” FIB-enhanced etching uses a reactive gas in combination with the FIB to increase the number of surface atoms removed by each impinging ion. Such a process is described, for example, in U.S. Pat. No. 5,188,705 to Swanson et al. for “Method of Semiconductor Device Manufacture.” In deposition and etching, the reactive gas is adsorbed onto the specimen surface and reacts in the presence of the ion beam. The rate of material removal or deposition depends on the number of ions striking the target surface, the rate at which gas molecules are adsorbed by the surface, and the number of atoms removed or deposited by each ion.
To produce smaller features, users have demanded ever higher resolution of FIB systems. Higher resolution implies a smaller diameter ion beam, often on the order of one tenth of a micron or less. Small diameter ion beams are typically Gaussian shaped beams produced by forming an image of the ion source on the target surface. Another method of forming small diameter beams, used in ion beam lithography, includes forming an image of an aperture onto the target. Such aperture imaging techniques are described, for example, in H. N. Slingerland, “Optimization of a Chromatically Limited Ion Microprobe,” Microelectronic Engineering 2, pp. 219-226 (1984) and J. Orloff and L. W. Swanson, “Some Considerations on the Design of a Field Emission Gun for a Shaped Spot Lithography System,” Optik, 61, No. 3, pp. 237-245 (1982). Such small diameter beams typically contain fewer ions, that is, have a lower beam current, than larger diameter beams. The rate at which material is etched or deposited by such beams is reduced because the total number of ions in the beam is reduced.
With improvements in ion beam producing technology, the beam current density, that is, the current per unit area, has been increased. As the ion beam dwells on each surface site or pixel in its scan pattern, the adsorbed gas molecules are reacted and removed by the high current density beam faster than they can be replenished by the broadly aimed gas jet. This phenomena is known as “overmilling” and applies to both FIB etching and depositing when the gas flux is insufficient to support the ion flux. This extensive gas removal makes the ion beam induced etch or deposition less efficient than if a higher density of adsorbed molecules were present on the surface. In deposition, the low density of the adsorbed gas not only reduces the deposition rate, but also some of the material already deposited may be etched away by the ion beam.
The FIB deposition rate or FIB enhanced etch rate may be limited by either the beam current at the required resolution or the supply of gas molecules. Platinum deposition is a particularly difficult case, where the beam current, and correspondingly the current density, must be limited to prevent overmilling of the adsorbed platinum-organic material.
Because the gas jet is much broader than the sub-micron ion beam, merely increasing the flow of gas toward the specimen is in some cases insufficient to provide an adequate supply of gas molecules adsorbed near the impact point of the ion beam. Moreover, most of the gas injected into the vacuum chamber is not reacted. The gases used in FIB etching and deposition are often corrosive, and unreacted gas molecules, which increase as the flow rate is increased, can degrade components in the vacuum system. Large increases in the gas flow rate would also adversely affect the vacuum required to maintain the ion beam.
One solution to the overmilling problem is to reduce the beam current. This solution reduces the rate of deposition or etching and results in unacceptably long processing times, particularly when a large amount of material is to be deposited or etched.
Another solution to the overmilling problem is to increase the scanning rate, that is, decrease the dwell time of the ion beam at each surface pixel in the scan pattern to move the beam to a new position before the adhered gas is exhausted. There are physical limits, however, to how fast the beam can be accurately scanned. Because the gallium ions in the beam typically travel at about three tenths of a millimeter each nanosecond and the ion column deflection plates are tens of millimeters long, the time for the ions to traverse the deflection plates becomes significant at short dwell times and limits the scan speed. Currently, the minimum pixel dwell time used is about one hundred nanoseconds. Thus, with some gases the ion beam cannot be scanned sufficiently fast to change locations before the adhered gas molecules are extensively reacted.
Another solution is to use a weakly defocused ion beam system that produces a broader, lower current density beam. Such beams cannot, however, produce the sharp edges and high resolution required in modern applications. The one dimensional current density profile (current density along a single axis through the center of the beam) of most focused ion beams is approximately Gaussian, or bell shaped. Most of the ions are in a center portion of the beam and the number of ions decreases gradually towards the beam edges. This non-uniform beam distribution causes uneven etching and deposition. The broad beam has a broadly tapering edge that results in unacceptably sloped, rather than sharp, vertical edges on etched or deposited features.
Thus, a solution to the overmilling problem that provides high resolution and high processing rate is required.